Cheating is actually having a SEXUAL relationship with anyone but the other in your relationship. That means oral, anal or vaginal sex. That means the girls cant do anything to guys but give them hand or boob jobs (without her mouth). If her mouth touches his dick, then its oral sex. If his dick enters her pussy or ass then its sex as well (vaginal and anal respectively). If he eats her out its still technically oral (cheating).
If they are close friends, they can however kiss, give hand jobs, eat each others "love juice", feel each other up and anything else that isn't sex.

EXCEPTIONS:
As with everything, there are some exceptions. They include...
-If you aren't close friends (it is cheating)
-If you have an open relationship (then NOTHING is really cheating anymore)
-If you and your significant other have other exceptions made (such as partially open relationship that specifically make it fine to do what you are doing)
-If you and your significant other have decided that the cheating boundary is more strict (then you MUST follow the new guide lines)

How does this work you might ask? Well you obviously haven't heard of a "romantic friendship". To qualify you and the other person must know each other well, you two must trust each other and you two must enjoy spending the time together.
Romantic friendships are real. They are very uncommon today, but at one time they used to be everywhere.

Steven: "Alexa, I heard you were found making out with . your friend the other day. Is this true?"
Alexa: "Yes, we were."
Steven: "Why would you cheat on me?"
Alexa: "Oh, it isn't cheating. We have known each other . since grade 8 and we weren't having a sexual . relation"
Steven: "So what is it if it isn't cheating?"
Alexa: "Its called a romantic friendship."
Steven: "So its okay to do that sort of thing now?"
Alexa: "Yes, In fact it has been for a long time."
Steven: "Okay, that sounds good."
Alexa: "Just remember, you can't have sex in any form with . them!"